1. Field Of The Invention
The present invention relates to gas phase polymerization processes utilizing multi-stage fluid beds, and particularly, but not by way of limitation, to such processes adapted for the production of olefin polymers such as polyethylene or polypropylene.
2. Description Of The Prior Art
In a typical presently operating gas phase ethylene or propylene polymerization plant, polymerization is done in a single fluidized bed and the effluent is cooled and recycled. In the case of ethylene polymerization, the recycle rate is fifty to sixty pounds of ethylene per pound of polymer produced. The purpose of this recycling is to remove the heat of reaction of the polymer which is approximately 1460 btu/lb. Such single bed processes have significant disadvantages. First, elaborate and expensive recycle equipment is required and the recycled stream may become contaminated. Second, in a single fluidized bed, catalyst is injected and mixes with the polymer, and therefore some of the catalyst is discharged along with the polymer before it has a chance to make much if any polymer. Therefore, efficiency is limited and the product is very heterogeneous.
Such a single stage fluidized bed reactor for olefin polymerization is disclosed in U.S. Pat. No. 4,003,712 to Miller.
It is also known that multiple stage vertically stacked fluidized bed reactors can be utilized for olefin polymerization. U.S. Pat. No. 2,936,303 to Goins, and assigned to Phillips Petroleum Company, the assignee of the present invention, discloses such a multi-stage polymerization process. Gaseous olefins are fed into the lower stage, and a fine particulate catalyst is fed into the upper stage. The gaseous stream flows upward and is steadily converted to polymer particles which flow downward by gravity through the stages of the reactor.
U.S. Pat. No. 2,467,802 to Barr discloses a multistage reactor having stages of successively decreasing diameter from bottom to top, but this disclosure is not involved with polymerization processes. Instead it deals with synthesis of hydrocarbons. The two processes are very different. In the present invention the gaseous monomer is injected into the lowermost stage, and as it flows upward it is continuously converted to solid polymer particles which in fact make up the fluidized bed and then flow downward through the series of fluidized beds and exit from the lowermost bed. In the hydrocarbon synthesis process of the '802 patent, on the other hand, the gases supplied to the lowermost stage react to convert into hydrocarbons which continue their upward flow. The downward flow through the fluidized beds of the '802 patent is solely catalyst material which becomes increasingly deactivated as it travels downward through the reactor.